Donna Kubik- Ultra high energy cosmic rays

8/3/2019 Donna Kubik- Ultra high energy cosmic rays

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Ultra high energy cosmic rays

Donna Kubik

Spring, 2005


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Cosmic accelerators

Thanks to Angela Olinto for

invaluable references and to

Jeff Wilkes for getting me into

all this in the first place!

Injection, acceleration, propagationin SN1987A


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References

1. Ultra High Energy Cosmic Rays: The theoretical challenge, A. V.Olinto, Phys.Rept.333 (2000) 329-348

astro-ph/0002006

2. Origin and Propagation of Extremely High Energy Cosmic Rays,Pijushpani Bhattacharjee and Guenter Sigl, Phys.Rept. 327(2000) 109-247

astro-ph/9811011

3. Introduction to Ultrahigh Energy Cosmic Ray Physics, PierreSokolsky, Addison-Wesley, 1989

4. High Energy Cosmic Rays, Todor Stanev, Springer, 2004


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Cosmic accelerators

Notice the similar geometry

of cosmic ray accelerators, like

SN1987A, and the manmade

accelerator at Fermilab ;) !


Injection, acceleration, propagationin a manmade accelerator


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Cosmic accelerators

Energy spectra of cosmic rays

have been shaped by a

combination of injection,

acceleration and propagation


Injection, acceleration, propagationin a manmade accelerator


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Cosmic accelerators

These processes are modified

by energy losses, collisions,

and perhaps leakage from the

galaxy, and they all depend on

particle energies.

Synchrotron loss in manmade accelerators

Synchrotron emission from high energyelectrons in M87


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Cosmic ray detection

The cosmic ray spectrum has

been measured over a wide

range of energy

The method of CR detectiondepends on the energy of the

particles


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Direct detection


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Indirect detection

> 0.1x1015 eV

Flux is too low for directdetection

Extensive air showers(EAS)

Ground arrays

Atmospheric light Cherenkov

Nitrogenscintillation(UV)


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EAS

The atmosphere above thearea of the EAS array servesas a natural transducer andamplifier for the detector.

The primary energetic cosmicray collides with a nucleus inthe air, creating manysecondary particles whichshare the original energy.


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EAS

The secondary particles alsocollide with nuclei in the air,creating a new generation ofstill more particles thatcontinue the process.

This cascade, called an"extensive air shower, arrivesat ground level with billions ofenergetic particles that can bedetected over approximately10 square kilometers.


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Auger Observatory

Above 1019 eV, the arrival rateis only 1 particle per squarekilometer per year.

Cosmic rays with energies

above 1020 eV have anestimated arrival rate of only1/km2 per century!

Need huge area

i.e. Auger Observatorydetection area of 3000 squarekilometers Click on image to see EAS animation of

Pierre Auger Cosmic Ray Observatory Array

(Must be online to view animation)

Animation from Auger website http://www.auger.org/photos/AnimGif.html


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Composition

The particle identity isdetermined by the muoncontent of the shower inground arrays and the depth ofshower max in fluorescence

detectors

But this is difficult analysis athigh energies

Need more data


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The cosmic ray spectrum Part 1

< 1 GeV

Temporally correlated withsolar activity solar origin

1 GeV to Knee

Galactic origin (SNRshocks)

Knee to Ankle

Larger shocks (galacticwinds) and SN explosions

into stellar winds

Graph from [2]


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The cosmic ray spectrum Part 2

Ankle (>1019.5)

Drastic change is slope

Crossover from Galactic toextragalactic origin?

100s of events >1019 eV

20 events above 1020 eV

AGASA

Flys Eye

HIRES

Highest-energy event is3.2x1020 eV

Graph from [2]


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GZK cutoff Part 1

The many events that have beenobserved above 7 x 1019 eV withno sign of the Greisen-Zatsepin-Kuzmin (GZK) cutoff was notexpected

If the Ultra High Energy CosmicRays (UHECRs) are protons,nuclei, or photons fromextragalactic sources, an energycutoff should be present

Protons with an initialE > 1020 eV

from >100 Mpc should appear at alower energy due to energy loss tothe CMB.

Graph from [2]


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GZK cutoff Part 2

The CMB has a blackbody spectrum with T=2.7 K corresponding to

mean energy of 6.34x10-4 eV [4]. A proton of high enough energy

(> 7x1019 eV) will interact inelastically with CMB photons producing

pions via

0!"

!"

pp

np

#+

#++


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GZK cutoff Part 3

With each collision, the proton would lose roughly 20% of its energy

This only happens for cosmic rays that have at least 6 x 1019 eV ofenergy, and this is the predicted GZK cutoff.

So if cosmic rays were given an initial energy greater than that, theywould lose energy in repeated collisions with the cosmic microwavebackground until their energy fell below this cutoff.

However, if the source of the cosmic ray is close enough, then it willnot have made very many collisions with microwave photons, and its

energy could be greater than the GZK cutoff.

This distance is about 150 million light years.


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Calculation of the required UHECR

proton energy for pion photoproduction

The approach is to calculate theproton energy, Ep, required forpion photoproduction usingconservation of 4-momenta, P.

Considering the left hand side inthe lab frame and the right handside in the center-of-mass frame,where

Ep = UHECR proton energy(the unknown)

E= average CMB photonenergy = 6.34x10-4 eV [4]

mp= 938.27 MeV/c2 m

0= 134.97 MeV/c2

P= 4-momentum

Conclusion: Ep~2x1020 eV

See Ref 4 p221 (Stanev)

( )( )

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massofCenterLab

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Calculation of the gamma wall energy

The approach is to calculate thephoton energy, E

, required for

e+e- production on the CMB usingconservation of 4-momenta, P.

Considering the left hand side inthe lab frame and the right handside in the center-of-mass frame,where

E

= gamma ray (gamma wall)photon energy

Ecmb= average CMB photonenergy = 6.34x10-4 eV [4]

me+= me-= 0.511 MeV/c2 P= 4-momentum

Conclusion: E~8.2x1014 eV

See Ref 4 page 264 (Stanev)

( )( )

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[ ]

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GZK cutoff Part 4

Above the threshold, multiple pion production becomes important.The +will decay via

Thus, for each such interaction there will be three neutrinos, as wellas gamma rays from the decay of the :

+++

!! ee

""""#

!!" #0


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GZK cutoff Part 5

The presence or absence of the GZK cutoff is fundamental to

understanding the sources and nature of the propagation of UHECR


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Possible sources

Sources of high energy CR arethought to be powerful radiogalaxies

But they are located>>150 Mpc from Earth

This is a major problem if theparticles are conventionalparticles as nucleons or heavy

nuclei

They should lose energy viathe GZK effect The neighboring superclusters

(Earth is at center of image,

in the Virgo Supercluster)

100 MLY

30.6 Mpc


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Possible sources

Mean free path ~ few Mpc

This process limits CRsources to


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Isotropy

Regardless of whether thesource of the UHECR isGalactic or extragalactic, atthese high energies, theparticle paths should not be

significantly affected bymagnetic fields, so their arrivaldirection should point back totheir source

But the arrival directions are

isotropic

Note, B= uG in Galaxy,


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The Challenge

The absence of a GZK cutoffand the isotropy of arrival directionsare two of the main challenges that models for the origin of UHECRsface

Maybe the source resides in an extended Galactic halo

That eases the difficulty of the lack of GZK cutoff but is a challengeto acceleration mechanisms

There are 2 scenarios popularly proposed for the origin of UHECRs:

Bottom-up

Top-down


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Two scenarios Part 1

Bottom-up

Astrophysical acceleration

Top-down

Physics beyond the standardmodel


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Bottom-up

Main hurdle is the need tofind a way to produce the

high energies

Top-down

Main hurdle is to account

for the high flux


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Bottom-up

Pro: The highest energyevent (3.2 x 1020 eV)

argues for the existence ofZevatrons (ZeV=1021 eV),

Con: But even the mostpowerful astrophysicalobjects, as radio galaxies

and AGNs, can barelyaccelerate chargedparticles to 1020 eV

Top-down

Pro: Decay of very highmass relics from the early

universe; no accelerationmechanism needed

Con: The dynamics oftopological defectgeneration and evolution

generally selects thepresent horizon scale asthe typical distancebetween defects whichimplies a very low flux


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Bottom-up

Statistical acceleration

Fermi acceleration

Acceleration in shocksassociated with SNremnants, AGNs, radiogalaxies

Direct acceleration

Assumes existence of a

strong electromagnetic field Acceleration in strong E

fields generated byrotating neutron starswith high surface Bfields

Top-down

Relics of the very earlyuniverse (topological defectsor superheavy relics) produced

after the end of inflation candecay today and generateUHECR

ZeV energies are not achallenge since symmetry-breaking scales at the end of

inflation are >> 1021 eV (typicalparticle masses between 1022 -1025 eV)


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Hillas plot Part 1

Acceleration of UHECR inastrophyiscal plasmasoccurs when large-scalemacroscopic motion, suchas shocks ad turbulentflows, is transferred toindividual particles

The max energy can beestimated by the gyroradiusof the particles contained inthe acceleration region

Graph from [1]


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Hillas plot Part 2

For a given strength, B,and coherence length L, ofthe field embedded in theplasma, Emax=ZeBL

Hillas plot shows knownastrophysical sources withreasonable BL

B

L

Graph from [1]


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GRB

Can GRBs be a source ofUHECR?

GRB and UHECR are bothdistributed isotropically in

the sky

Average rate of gamma rayenergy emitted by GRBs iscomparable to the energygeneration rate of UHECRof energy > 1019 eV

GZK still a problem

Isotropic distribution of GRB


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Hybrid Models

Uses physics beyond the standard model in a bottom-up approach,hence termed Hybrid

Uses Zevatrons to generate UHECR that are particles other thanprotons, nuclei, and photons,

One example: Neutrino masses ~ 1eV, the relic neutrinobackground will cluster in halos of galaxies and clusters of galaxies.High energy neutrinos accelerated in Zevatrons can annihilate onthe neutrino background and form UEHCR through the hadronic Zdecay.

Neutrinos do not suffer from GZK losses, so the Zevatrons can belocated much farther away


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Power law

Power law exists over many

decades

Source must generate a power

law spectrum

Graph from [2]


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Composition

The composition of theUHECR could helpdiscriminate betweenproposed sources

Galactic disk models need toinvoke heavier nuclei (Fe) tobe consistent with the isotropicdistribution

Extragalactic models favorproton primaries

Photon primaries morecommon among top-downscenarios

Graph from [2]


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The future

The new experiments, HIRES,

the Auger Observatory, and

the OWL-Airwatch satellite will

improve data on the spectrum

and spatial distribution, andcomposition of UHECR.

The lack of a GZK cutoff

should become apparent with

Auger and, if so, most

extragalactic Zevatrons maybe ruled out.

Graph from [2]


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The future

The observed spectrum will

distinguish Zevatrons or top-

down model by testing power

las vs. QCD fragmentation fits

The correlation of arrival

directions of UHECR with

some known structure as the

Galaxy, Galactic halo, Local

Group, or Local Supercluster

would be key in differentiatingbetween different models

Graph from [2]

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Donna Kubik- Ultra high energy cosmic rays